1. Field of the Invention
This invention relates to a resonance type electric power conversion apparatus and method, and more particularly to a soft switch upon elimination of recovery current and upon turning on of a main switching device of a resonance type electric power conversion apparatus of a voltage step-up circuit, a voltage step-down circuit and so forth.
2. Description of the Related Art
In a hybrid automobile, a fuel cell vehicle, an electrically driven vehicle and like vehicles, driving force is generated by a motor generator and transmitted to an axle. In order to obtain driving force optimum to a driving condition of the vehicle, a power supply voltage of a direct current (dc) power supply is boosted to a desired voltage by a voltage step-up circuit, and the driving force of the motor generator is obtained based on the boosted voltage. Further, a voltage generated by the motor generator is stepped down to a desired voltage by a voltage step-down circuit, and do power is regenerated based on the stepped down voltage.
In order to implement high output power and a high efficiency, it is necessary to reduce the switching loss as far as possible and use a higher frequency to reduce the scale as far as possible. To this end, a technique is required for eliminating recovery current of an output power diode or a return diode upon soft switching such as voltage-zero current switching (ZVCS) or zero-voltage switching (ZVS) upon switching on/off of a switching device and upon switching on of a main switching device. The recovery current here signifies current flowing through the main switching device and originating from charge accumulated in the output power diode or the like when the main switching device is turned on to reversely bias the output power diode or the like.
As a resonance type electric power conversion apparatus, a SAZZ (Snubber Assisted Zero voltage and Zero current transition) voltage step-up circuit, a ZVT (Zero Volt Transition) voltage step-up circuit and so forth have been proposed. Such electric power conversion apparatus include a main circuit including a main reactor, a main switching device, an inverse-parallel diode connected in inverse-parallel to the switching device and an output power diode, and an auxiliary circuit including an auxiliary reactor, an auxiliary switch and an auxiliary capacitor for forming a recovery current elimination circuit for discharging charge accumulated in the output power diode and a partial resonance circuit for discharging charge accumulated in the auxiliary capacitor.
The auxiliary switching device is turned on to output charge accumulated in the output power diode through the recovery current elimination circuit (MODE 1). Charge accumulated in the auxiliary capacitor is discharged through the partial resonance circuit (MODE 2). When the auxiliary capacitor completes the charging and the charge disappears from the auxiliary capacitor, back electromotive force is generated in the auxiliary reactor and the inverse-parallel diode is turned on, and consequently, current flows through the inverse-parallel diode. At this point of time, the main switching device is turned on (MODE 3). When the main switching device is turned on, current flows from the power supply to the main switching device through the main reactor and magnetic energy is accumulated in the main reactor (MODE 4). The main switching device is turned off to charge the auxiliary capacitor (MODE 5). When the voltage of the auxiliary capacitor becomes equal to the output voltage, the output power diode is turned on, and power is supplied to the load side through the output power diode (MODE 6).
Conventionally, the time after the auxiliary switching device is turned on until the main switching device is turned on is set to a fixed period of time irrespective of the output power of the voltage step-up circuit and so forth. Also in the resonance type electric power conversion apparatus as a voltage step-down circuit, the time after the auxiliary switching device is turned on until the main switching device is turned on is set to a fixed period of time irrespective of the output power of the voltage step-down circuit and so forth similarly.
Japanese Patent Laid-open No. 2001-309646 is available as a conventional art document relating to soft switching of the voltage step-up circuit described above.
However, conventional voltage step-up circuits have the following problem in control thereof. As described above, the delay time after an auxiliary switching device of a voltage step-up circuit or the like is turned on until a main switching device is turned on is set to a fixed period of time as described above. However, in the MODE 1, the time (recovery current eliminating time) for eliminating charge accumulated in the output power diode using the recovery current elimination circuit is a variable period of time which depends upon the output voltage, input voltage, current (output current) flowing to the main reactor and so forth. Since the output voltage, output current and so forth are fluctuated by a load to a motor generator to which the voltage step-up circuit or the like is connected and the like, the recovery current elimination period fluctuates as time passes. Conventionally, since the delay time after the auxiliary switching device is turned on until the main switching device is turned on is set to a fixed period of time, the delay time is not an optimum period of time.
Therefore, where the delay time is longer than optimum time, current flows excessively as much to the auxiliary switching device. Consequently, there is a problem that the efficiency is deteriorated by resistance loss in the auxiliary switching device. Further, although the inverse-parallel diode is turned on and current flows at a point of time when the charge accumulated in the auxiliary capacitor disappears, a different circuit loop is produced, which blocks soft switching of the main switching device, resulting in a problem that the switching loss increases. This gives rise to a problem that the efficiency of the electric power conversion apparatus is deteriorated.
On the other hand, where the delay time is shorter than the optimum time, the discharging of the auxiliary capacitor does not come to an end. Consequently, the charge remains, and the inverse-parallel diode is not turned on and the MODE 3 is not entered. Therefore, the main switching device switches not by soft switching but by hard switching, resulting in a problem that the switching loss is high. This gives rise to a problem that the efficiency of the electric power conversion apparatus is deteriorated.
Meanwhile, in the conventional technique of Japanese Patent Laid-open No. 2001-309646, the main switching device 110 is controlled so as to turn on in response to detection that the voltage across the auxiliary capacitor 141 decreases to zero. However, the operation condition of the voltage step-up circuit varies in response to variation of the load 150, and for example, in a low load region, charge remains in the auxiliary capacitor 141 and the voltage across the auxiliary capacitor 141 does not reduce to zero. Therefore, the conventional technique described has a problem that the control of the voltage step-up circuit is disabled in the low load region.